Yet nearly a decade before his 1973 film “Enter the Dragon” inspired Americans to enroll in traditional martial arts such as karate and kung fu, Mr. Lee had abandoned his own kung fu training and embraced an amalgam of fighting styles, taking whatever he found useful from the likes of judo and Western boxing, among others.

Finally, Americans seem to be catching up.

Thanks to the success of the Ultimate Fighting Championship, a series of competitions based in Las Vegas, mixed martial arts, a sport that combines techniques from a host of fighting styles — from Brazilian jiujitsu to Thai kickboxing — has become the latest martial arts craze in the United States.

What defines mixed martial arts varies. But experts said a legitimate school should teach several basic skills like how to strike, how to wrestle and how to fight on the ground.

“When we were young, kids took karate or tae kwon do,” said Dana White, the president of the Ultimate Fighting Championship.

“Mixed martial arts is the new martial art.”

Or rather, the old new martial art.

In the opening scene of “Enter the Dragon,” Mr. Lee squares off against an opponent — the two of them dressed in tight-fitting shorts and black fingerless gloves — trading punches, kicks and throws until Mr. Lee applies a painful arm lock and forces his opponent to surrender.

At the time, the fight scene was considered revolutionary for its fusion of different styles; in the ’70s, cross-training in different martial arts was taboo, martial arts experts said.

But today, from Mr. Lee’s moves to his uniform, the scene bears a noticeable resemblance to what is going on in mixed martial arts gyms across the country.

“He was like 50 years ahead of his time,” said Dan Inosanto, one of Mr. Lee’s former students and the head instructor at the Inosanto Academy in Marina del Rey, Calif.

It wasn’t until 1993 that the first professional mixed martial arts bouts came to the United States. Yet until several years ago, the sport was largely viewed as brutal and a fringe phenomenon, once called “human cockfighting” by Senator John McCain. By 2000, it was banned in most parts of the country.

Today, after cleaning up its act, mixed martial arts is legal and regulated in nearly every state. And thanks to savvy marketing and a popular reality show, the Ultimate Fighting Championship, the sport’s premier brand, has transformed mixed martial arts into a mainstream spectator sport.

But Americans aren’t only watching mixed martial arts, they are increasingly practicing it for fun and fitness as well. Century Martial Arts, a leading brand of martial arts equipment, said revenue from its mixed martial arts products doubled last year.

Meanwhile, mixed martial arts schools are popping up across the country. Schools that previously offered only karate or tae kwon do are now advertising instruction in M.M.A., the sport’s popular acronym.

Frank Silverman, the president of the Martial Arts Industry Association, a trade group, said the number of mixed martial arts representatives at his group’s largest trade show has tripled over the last two years.

Later this month, the Ultimate Fighting Championship plans to open a chain of gyms, and many mixed martial arts instructors said they have seen increased interest from students who want to start clubs at the high school and college levels.

The major influx of enrollment, martial arts instructors said, has come from the Ultimate Fighting Championship’s biggest demographic, men age 18 to 34.

“People see mixed martial artists and look at the shape these guys are in,” said Daniel Schulmann, the owner of Tiger Schulmann’s Mixed Martial Arts, a chain with 48 schools in five states.

“Fighters have power like a power lifter but endurance like a runner,” he said. “And they’re flexible.”

Some students like Danny Rangel, 21, who trains at Camp Undefeated, a mixed martial arts school in Manhattan, want to become professional fighters.

“I’m looking to compete,” Mr. Rangel said in an interview. “The ultimate goal one day would be the U.F.C.”

But the majority of people simply want an alternative to the gym and some self-defense experience, martial arts experts said.

In fact, mixed martial arts has acquired such a good reputation as a workout that even professional athletes from more established sports are participating. Most recently, Glen Davis, 23, a forward with the Boston Celtics, has begun working out at Wai Kru, a mixed martial arts school in Boston, in an effort to lose weight.

“He’s down 15 pounds at least,” said John Allan, the school’s chief instructor.

Yet for people like Mr. Davis, instructors said they have to draw a fine line between training their students like fighters and preventing injuries.

“Our motto is ‘Come for the mixed martial arts workout,’ ” said Jorge Gurgel, a professional fighter and the owner of a number of mixed martial arts academies, most of which are in the Midwest.

Although Mr. Gurgel’s schools boast Ultimate Fighting Championship veterans, he said some of his classes still appeal to women and older men.

“They can do the workout without having to fight,” he said.

That freedom to participate and get in shape — without getting a black eye — is what appealed to Hillary Gilmore, 26, who trains at Mr. Schulmann’s school in Manhattan.

Yet her friend, Carissa Beal, 27, admitted that she initially felt uncomfortable with grappling, an aspect of mixed martial arts that often involves sitting between a person’s legs or lying on top of them.

“If you’re doing it with a stranger, it’s really awkward,” she said.

Maybe so. But more than 30 years after Mr. Lee sparked the first martial arts revolution in the United States, Americans have finally accepted his message, however awkward it may seem in practice.